Adding a lock to cabinet or drawer can be a sure sign to a thief that they are on the right track. Ideally, you could secure the cabinet without any visible indication that it is locked. That is the idea behind the Covert from Quirky.
The Covert is a fairly inexpensive, easily installed drawer and cabinet lock that leaves no visible indication of the presence of a lock. The lock is operated with a magnet that slides a bolt into a lock plate. This Quirky can likely be bypassed with brute force (like any lock) but hopefully, its discreet design prevents anyone from even getting to that point.
Check out Covert on Quirky.com.
Pretty sure this product already exists. Most magnetic baby locks do the same thing.
This is what I used for my kids and have thought of using for a hidden gun cabinet:
http://www.toysrus.com/product/index.jsp?productId=4449506